The escalating requirements for high density and performance associated with ultra large scale integration semiconductor wiring require responsive changes in interconnection technology. Such escalating requirements have been found difficult to satisfy in terms of providing a low RC (resistance capacitance) interconnection pattern, particularly wherein submicron vias, contacts and trenches have high aspect ratios due to miniaturization.
Conventional semiconductor devices typically comprise a semiconductor substrate, typically undoped monocrystalline silicon, and a plurality of sequentially formed inter-layer dielectrics and patterned metal layers. An integrated circuit is formed containing a plurality of conductive patterns comprising conductive lines separated by interwiring spacings, and a plurality of interconnect lines, such as bus lines, bit lines, word lines and logic interconnect lines. Typically, the conductive patterns on different layers, i.e., upper and lower layers, are electrically connected by a conductive plug filling a via opening, while a conductive plug filling a contact opening establishes electrical contact with an active region on a semiconductor substrate, such as a source/drain region. Conductive lines are formed in trenches which typically extend substantially horizontally with respect to the semiconductor substrate. Semiconductor "chips" comprising five or more levels of metallization are becoming more prevalent as device geometries shrink into the deep submicron range.
A conductive plug filling a via opening is typically formed by depositing an inter-layer dielectric on a patterned conductive (metal) layer comprising at least one metal feature, forming an opening in the inter-layer dielectric by conventional photolithographic and etching techniques, and filling the opening with a conductive material, such as tungsten (W). Excess conductive material on the surface of the inter-layer dielectric is removed by chemical-mechanical polishing (CMP). One such method is known as damascene and basically involves the formation of an opening which is filled in with a metal. Dual damascene techniques involve the formation of an opening comprising a lower contact or via opening section in communication with an upper trench opening section, which opening is filled with a conductive material, typically a metal, to simultaneously form a conductive plug in electrical contact with a conductive line.
High performance microprocessor applications require rapid speed of semiconductor circuitry. The speed of semiconductor circuitry varies inversely with the resistance and capacitance of the interconnection pattern. As integrated circuits become more complex and feature sizes and spacings become smaller, the integrated circuit speed becomes less dependent upon the transistor itself and more dependent upon the interconnection pattern. Miniaturization demands long interconnects having small contacts and small cross-sections. As the length of metal interconnects increases and cross-sectional areas and distances between interconnects decrease, the RC delay caused by the interconnect wiring increases. If the interconnection node is routed over a considerable distance, e.g., hundreds of microns or more, as in submicron technologies, the interconnection capacitance limits the circuit node capacitance loading and, hence, the circuit speed. As design rules are reduced to about 0.18 micron and below, the rejection rate due to integrated circuit speed delays severely limits production throughput and significantly increases manufacturing costs. Moreover, as line widths decrease, electrical conductivity and electromigration resistance become increasingly important.
As device geometries shrink and functional density increases, it becomes increasingly imperative to reduce the capacitance between metal lines. Line-to-line capacitance can build up to a point where delay time and cross talk may hinder device performance. Reducing the capacitance within multi-level metallization systems will reduce the RC constant, cross talk voltage, and power dissipation between the lines.
One way to increase the speed of semiconductor circuitry is to reduce the resistance of a conductive pattern. Conventional metallization patterns are typically formed by depositing a layer of conductive material, notable aluminum or an alloy thereof, and etching, or by damascene techniques wherein trenches are formed in dielectric layers and filled with conductive material. The use of metals having a lower resistivity than aluminum, such as copper, engenders various problems which limit their utility. For example, copper readily diffuses through silicon dioxide, the typical dielectric material employed in the manufacture of semiconductor devices, and adversely affects the devices. In addition, copper does not form a passivation film, as does aluminum. Hence, a separate passivation layer is required to protect copper from corrosion.
The dielectric constant of materials currently employed in the manufacture of semiconductor devices for inter-layer dielectrics (ILD) spans from about 3.9 for dense silicon dioxide to over 8 for deposited silicon nitride. Prior attempts have been made to reduce the interconnect capacitance and, hence, increase the integrated circuit speed, by developing dielectric materials having a lower dielectric constant than that of silicon dioxide. New materials having low dielectric constants, such as low dielectric constant polymers, teflon and porous polymers have been developed. There has been some use of certain polyimide materials for ILDs which have a dielectric constant slightly below 3.0.
Recent attempts have also resulted in the use of low-density materials, such as an aerogel, which has a lower dielectric constant than dense silicon oxide. The dielectric constant of a porous silicon dioxide, such as an aerogel, can be as low as 1.2, thereby potentially enabling a reduction in the RC delay time. However, conventional practices for producing an aerogel require a supercritical drying step, which increases the cost and degree of complexity for semiconductor manufacturing. Moreover, the use of an aerogel results in a semiconductor device which lacks sufficient structural integrity.
Prior attempts to reduce parasitic RC time delays also include the formation of various types of air gaps or bridges. See, for example, Lur et al., U.S. Pat. No. 5,413,962, Jeng, U.S. Pat. No. 5,708,303 and Saul et al., UK Patent GB2,247,986A. However, the removal of ILD material becomes problematic in various respects. Firstly, the removal of ILD material adversely impacts the structural integrity of the resulting semiconductor device rendering it unduly fragile. Secondly, the removal of ILD material results in a significant reduction in electromigration resistance of the conductors due to exposed free surfaces.
Accordingly, there exists a need for a semiconductor device having reduced parasitic RC time delays with reduced internal capacitance without sacrificing structural integrity and/or electromigration performance.